I have two existing wood burning stoves - fireplace inserts - that I would like to replace with natural gas units. What are my options which would meet the following criteria - use existing fireplace openings, high efficient heat output. Also I'm confused about the use of BTU's, which I had thought of as an indication of heat output, but you use it in terms of heat consumption where it seems that a unit can use lots of BTU but not produce much heat for the home (which I need).

Answer:

There is BTU input and BTU output. A bonfire or blaze in an open fireplace puts out lots of BTU's, but they all go up the chimney. The term "efficiency" is used to describe how much of the heat is kept in the house. For instance, a fire burning at 90,000 BTU, but only putting 9,000 in the home would be 10% efficient.

So, you are looking for either a sealed gas insert or vent free logs. There is much discussion of both of these products on the info page at: http://hearth.com/what/specific.html and on the Q and A database: http://chi.hearth.com/search.html